- The gravitational field strength, g at a point describes how strong or weak a gravitational field is at that point
- g in a radial field (such as a planet) is calculated using the equation:
- g = gravitational field strength (N kg-1)
- G = Newton’s Gravitational Constant
- M = mass of the body producing the gravitational field (kg)
- r = distance from the mass where you are calculating the field strength (m)
- Gravitational field strength, g, is a vector quantity
- The direction of g is always towards the centre of the body creating the gravitational field
- This is the same direction as the gravitational field lines
- On the Earth’s surface, g has a constant value of 9.81 N kg-1
- However, outside the Earth’s surface, g is not constant
- g decreases as r increases by a factor of 1/r2
- This is an inverse square law relationship with distance
- When g is plotted against the distance from the centre of a planet, r has two parts:
- When r < R, the radius of the planet, g is directly proportional to r
- When r > R, g is inversely proportional to r2 (this is an ‘L’ shaped curve and shows that g decreases rapidly with increasing distance r)
- Sometimes, g is referred to as the ‘acceleration due to gravity’ with units of m s-2
- Any object that falls freely in a uniform gravitational field on Earth has an acceleration of 9.81 m s-2
The mean density of the Moon is 3/5 times the mean density of the Earth.
The gravitational field strength on the Moon is 1/6 of the value on Earth.
Determine the ratio of the Moon’s radius rM and the Earth’s radius rE.
Remember that r is always taken from the centre of mass of the object creating the gravitational field.
Also, make sure you’re comfortable with drawing the inverse square law graph of g against r, since this is a common exam question